Geological Influence on the Site Response of Bantul Earthquake at Yogyakarta Special Province, Indonesia
On May 27, 2006 a magnitude 6.2 earthquake struck Yogyakarta Special Province in Central Java. The earthquake designated the Bantul earthquake resulted in the deaths of 5,800 and left 200,000 families homeless. Over 280,000 buildings were damaged or destroyed. Housing quality was generally poor with no accounting for earthquake-resistant design. Housing and building materials were also of poor quality. In spite of the poor design and construction materials, local geological conditions were determined be a significant influence on the site response which affected the intensity of building damage. In addition, strong ground shaking was unusually long at 57 seconds. In response to the findings of the initial field reconnaissance a series of micro-tremor surveys supported by aerial and satellite image interpretations, ground penetration radar and magneto telluric surveys, as well as engineering geological site investigations were conducted. The objectives of the surveys and investigations were: to investigate various factors controlling the levels of site response which induced damage to homes and buildings; and, to produce a seismic hazard micro-zonation map. Provision of this micro-zonation map is crucial to support the enhancement of building code and landuse management in the earthquake prone area at Bantul. The Bantul area is located in a valley formed a graben. This valley was due to two major normal faults extending towards North East - South West. The east border of the valley is bounded major normal fault formed the Progo River, whilst at the west border another major normal fault formed the Opak-Oya Rivers. Drilling correlations of the bedrock (of andesitic breccia) in the valley, was found at a depth of about 40 m below the existing ground surface. Bedrock was covered by a layer of clay produced from paleo-swamps with various thicknesses from 1 to 2 m. After the clay deposition, the basin then was filled by loose fluvial sediments consisting of gravelly sand and clayey sand, with intercalation of laharic deposits consisting of bouldery-gravelly sand. Micro-tremor surveys conducted across the Bantul Basin, indicated that amplifications occurred were limited to zones of loose gravelly sand - clayey sand within the first of 30 m from the ground surface. The surveys identified four lateral zones where varying degrees of site response was identified. The zones were differentiated by levels of very high, high, moderate or low amplification. The surveys and investigations concluded that the most significant control of the site response is the local stratigraphic conditions, especially related to the type, density and thickness of sediments. Some other controls on the amplifications of site response are the existence of faults, the depth to bedrock, the shallow depth of the groundwater table, and the distance of the site to the earthquake epicenter.
AGU Spring Meeting Abstracts
- Pub Date:
- May 2007
- 7299 General or miscellaneous